The electronic structure of strongly correlated electron systems

强关联电子系统展示了一些最有趣的物理性质，可能在未来的电子设备、可持续能源、生物医学设备、交通甚至量子计算中应用。由过渡金属和稀土化合物(如氧化物)制成的材料通常具有极端的性能，如高温超导、高热电性能、高介电常数、胶体磁阻等。这些材料不同于像硅这样的传统半导体或像铝这样的简单金属，在这些材料中，电子在原子之间移动的电荷在决定应用中起着重要的作用。另一方面，过渡金属氧化物通常是电绝缘体和磁性的，因为电子之间有很强的排斥作用。然而，化学替代可以将这种磁性绝缘体转变为高温超导体(1988年诺贝尔奖)，并导致它们的性质发生其他戏剧性的变化。支配它们行为的基本物理学仍然是现代科学研究中最大的智力挑战之一。在这里，自旋和轨道自由度以及它们与电子的电荷自由度的相互作用起着至关重要的作用。需要新的实验方法对这些自由度敏感，并适用于纳米尺度的结构，如薄膜、界面和异质结构。

Strongly correlated electron systems exhibit some of the most interesting physical properties for possible future applications in electronic devices, sustainable energy, biomedical devices, transportation and perhaps even quantum computing. Materials made of transition metal and rare earth compounds such as the oxides are examples often exhibiting extreme properties such as high temperature superconductivity, high thermoelectric power, high dielectric constants, collassal magneto resistance and many more. These materials are different from conventional semiconductors like Si or simple metals like Al in which the charge of the electrons moving in amongst atoms plays the important role in determining the applications. Transition metal oxides on the other hand often are electrical insulators and magnetic because of the strong repulsive interaction between the electrons. However chemical substitution can turn such magnetic insulators into high temperature superconductors ( Nobel Prize 1988 ) and cause other dramatic changes in their properties. The basic physics governing their behaviour remains one of the biggest intellectual challenges of modern scientific research. Here the spin and orbital degrees of freedom and their interplay with the charge degrees of freedom of electrons play essential roles. New experimental methods are needed which are sensitive to these degrees of freedom and applicable to nanometer scale structures such as in thin films, interfaces and heterostructures.